1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic read head, more specifically to the use of an ultra-high vacuum sputtering system to form GMR layers which are inherently furnished with sub-monolayers of adsorbed oxygen (oxygen surfactant layers) for improved deposition properties.
2. Description of the Related Art
Early forms of magnetic read heads decoded magnetically stored data on media such as disks and tapes by making use of the anisotropic magnetoresistive effect (AMR) in magnetic materials such as permalloy. This effect was the change in the electrical resistance, r, of certain magnetic materials in proportion to the angle between the direction of their magnetization and the direction of the current flow through them. Since changing magnetic fields of moving magnetized media, such as magnetically encoded tapes and disks, will change the direction of the magnetization in a read head, the resistance variations of the AMR effect allows the information on such encoded media to be sensed and interpreted by appropriate circuitry.
One shortcoming of the AMR effect was the fact that it produced a maximum fractional resistance change, DR/R (where DR is the change in resistance between the magnetic material subjected to its anisotropy field, Hk, and the material subjected to zero field), which was only on the order of a few percent. This made the sensing process difficult to achieve with accuracy.
In the late 1980's and early 1990's the phenomenon of giant magnetoresistance (GMR) was discovered and soon applied to read head technology. The GMR effect derives from the fact that thin (≅20 angstroms) layers of ferromagnetic materials, when separated by even thinner (≅10 angstroms) layers of conductive but non-magnetic materials, will form ferromagnetic (parallel spin direction of the layers) or antiferromagnetic states (antiparallel spin direction of the layers) by means of exchange interactions between the spins. As a result of spin dependent electron scattering as electrons crossed the layers, the magnetoresistance of such layered structures was found to be significantly higher in the antiferromagnetic state than the ferromagnetic state and the fractional change in resistance was much higher than that found in the AMR of individual magnetic layers.
Shortly thereafter a version of the GMR effect called spin valve magnetoresistance (SVMR) was discovered and implemented. In the SVMR version of GMR, two ferromagnetic layers such as CoFe or NiFe are separated by a spacer layer, which is a thin layer of electrically conducting but non-magnetic material such as Cu. One of the ferromagnetic layers has its magnetization direction fixed in space or “pinned,” by exchange anisotropy with an antiferromagnetic layer deposited directly upon it. The remaining ferromagnetic layer, the unpinned or free layer, can respond to small variations in external magnetic fields such as are produced by moving magnetic media, (which do not affect the magnetization direction of the pinned layer because the exchange pinning strength exceeds the external fields), by rotating its magnetization direction. This rotation of one magnetization relative to the other then produces changes in the magnetoresistance of the three layer structure which is generally proportional to the cosine of the angle between the magnetization directions.
The spin valve structure has now become the implementation of choice in the fabrication of magnetic read head assemblies. Different configurations of the spin valve have evolved, including the bottom spin valve, wherein the pinned layer is at the bottom of the configuration and the top spin valve, wherein the pinned layer is at the top. In addition, the qualities of the spin valve have been improved by forming the pinned layer as a synthetic antiferromagnet, which is a layered configuration comprising two ferromagnetic layers separated by a non-magnetic coupling layer, wherein the ferromagnetic layers are magnetized in antiparallel directions.
The present challenge to the spin valve form of sensor is to make it suitable for reading recorded magnetic media with recorded densities exceeding 20 Gb/in2. This challenge can be met by making the free layer extremely thin, for improved resolution in the track direction, while not reducing DR/R, which is a measure of the sensor's sensitivity. One way of achieving this goal is by forming the spin valve on a seed layer, which is a layer of material whose purpose is to improve the crystalline structure of magnetic layers grown upon it. The present inventors have already shown that spin valves fabricated using a NiFeCr seed layer have a greatly enhanced GMR effect as measured by DR/R. Presently, the NiFeCr seed layer is becoming the industry standard for heads capable of reading densities exceeding 20 Gb/in2. The Cr composition of the seed layer in these heads is between 20 and 50 atomic percent, with the optimum value of DR/R obtained with 40 atomic percent. Lee et al. (U.S. Pat. No. 6,141,191) disclose a top spin valve using a NiFeCr seed layer wherein the atomic percentage of Cr is between 20 and 50%. Lee et al. report a DR/R for the configuration of 7.7% which is a significant improvement over similar sensor formed on Ta seed layers. According to Lee et al., the performance improvement is a result of being able to use NiMn as an antiferromagnetic pinning layer, which produces a high pinning field and high blocking temperature.
The present inventors have been using a bottom spin valve configuration (see below) meeting the requirements for reading recorded densities greater than 30 Gb/in2, yet its performance would be inadequate for reading area densities of 60 Gb/in2:NiCr(40%)60/MnPt100/CoFe15/Ru7.5/CoFe20/Cu18/OSL/CoFe—NiFe/Ru10/Ta10.In the above configuration the numbers (other than the 40%) refer to approximate layer thicknesses in angstroms. The NiCr seed layer has 40% atom percent Cr. MnPt is the antiferromagnetic pinning layer, CoFe/Ru/CoFe is a synthetic antiferromagnetic pinned layer, Cu is the spacer layer, OSL represents an oxygen surfactant layer formed on the Cu spacer layer, the surfactant layer being a sub-monolayer of oxygen deposited on the Cu surface by exposing the Cu layer to low-pressure oxygen in a separate chamber, CoFe—NiFe is a composite free layer formed by sequentially sputtering CoFe and NiFe on the surfactant layer and Ru/Ta is a composite capping layer wherein the Ru is used to prevent inter-diffusion between the Ta and the NiFe. The configuration provides a DR/R of 12.7% and a sheet resistance, Rs, of 19.6 ohm/sq.
In order to form sensor structures capable of reading densities in excess of 60 Gb/in2, it is necessary to reduce the track width of the sensor and to reduce the thickness of its free layer, while still retaining sufficient ratios of DR/R for adequate signal strength. Improvement of DR for a reduced trackwidth sensor can be obtained either by increasing sheet resistance of the sensor or DR/R or both. Increase in Rs and DR/R can be obtained by thinning the GMR film thickness. In this respect, thinning the Cu spacer layer is particularly advantageous because it has a very low Rs. For example, Cu spacer thickness can be reduced from 30 A to 22 A when the Ta seed layer is replaced by the NiFeCr (NiCr) seed layer. For such spin valves, DR/R is increased from 6.5% to 9.5%. An NiFeCr (NiCr) seed layer allows the synthetic pinned layer upon which the Cu spacer layer is deposited, to be grown with a much smoother surface, thereby enhancing the spin dependent specular reflectivity of conduction electrons at the inner surface of the Ru/CoFe layers within the synthetic pinned layer. When the surface of the Cu layer is treated with oxygen to form an oxygen surfactant layer (OSL), the thickness of the Cu layer can be further reduced to between 22 A and 18 A. With an oxygen dose of 10−4 torr-sec, the surfactant layer is less than a mono-layer thick. The formation of the OSL suppresses the interdiffusion at the Cu/CoFe interface when the CoFe free layer is deposited on the Cu layer. This increases the spin-dependent transmission of conduction electrons and suppresses scattering at the interface. The oxygen is highly mobile and has a strong tendency to diffuse out to the surface of the free layer to improve the specular reflectivity at the GMR outer surface of the CoFe—NiFe/Ru (the surface between the free layer and the capping layer). Because of the high mobility of oxygen in the Cu spacer layer, the oxygen surfactant layer can be formed at a variety of positions within the Cu layer during its formation. Thus, the surfactant layer can be formed at the Cu bottom surface where it meets the pinned layer (the CoFe/Cu interface), in the middle of the Cu layer, or at the Cu/CoFe interface with the free layer.
GMR sensors capable of reading area densities of approximately 45 Gb/in2, have been made using the following configuration:NiCr(40%)55/MnPt125/CoFe15/Ru7.5/CoFe20/Cu18/SL/CoFe10-NiFe20/Ru10/Ta10.Rs of this configuration is approximately 19.5 ohm/sq and DR/R is approximately 12.8%. Since DR=Rs×DR/R, DR=2.5 ohm/sq.
For recording densities greater than 60 Gb/in2, DR must be even greater. A practical approach to achieving this increase is to use a thinner free layer, such as CoFe 5-NiFe 20, together with a thinner layer of MnPt (eg. 100 A), to reduce current shunting through the MnPt. However, the thermal stability of a GMR sensor with these features has been found to be poor. The GMR configuration has, therefore, used CoFe10/NiFe15 and MnPt 125. In this configuration a DR=2.85 is obtained, which is marginally adequate for the 60 Gb/in2 sensor.
Up to the present time, GMR film stacks have been formed by sputtering in a sputtering system with a base pressure of approximately 10−8 torr, using Ar as the sputtering gas at a pressure of a few millitorr. This is a typical industry standard for the present generation of sputtered spin valves. Note in this regard that Sakakima et al. (U.S. Pat. No. 6,567,246), who will be discussed further, describes a sputtering process (column 16, lines 45-50) with 10−8 torr base pressure and an Ar pressure of 0.8 millitorr.
It is well known that thin film sputtering with an ultra-low sputtering pressure produces a smoother, flatter and denser film. Consequently, under these sputtering conditions, allows a thinner film to be formed with good qualities. Sputtering with low gas pressure of the sputtering gas (<1 millitorr) requires an ultra-high vacuum system and it is expected that by coupling the sputtering of GMR layers in such a system along with the addition of oxygen to the gas mixture to form a surfactant layer, should allow the formation of very high quality thin films. Presently, the design of sputtering systems has improved greatly and at least one commercially available manufacturing system (the Anelva C-7100), allows the production of a base pressure of 5×10−9 torr and an argon sputtering gas pressure as low as 0.1 millitorr. The present invention provides a GMR read sensor suitable for recorded densities greater than 100 Gb/in2 by making advantageous use of such new sputtering systems. As noted above, Sakakima is utilizing a very high vacuum sputtering system for forming MR elements with oxide magnetic films such as CoFe2O4, wherein such films are in the thickness range of several nanometers, significantly thicker than that envisioned in the present invention.
Inoue et al. (U.S. Pat. No. 6,414,825) teach a method for improving the thermal conductivity and hardness of shield gap films by sputtering BN, SiN and CN layers in the presence of oxygen and Ar. In short, there is evidence that the use of Ar and oxygen as sputtering gases can improve the qualities of a recording medium as well as the film layers in the sensor used to read that medium. Kanbe et al. (U.S. Pat. No. 6,221,508) teaches the use of Ar sputtering with the admixture of small percentages of other gases (including oxygen at approx. 10%) in forming recording media with reduced grain size for low-noise magnetic recording.
None of the prior art cited teaches the formation of GMR read sensors having ultra-thin layers that are rendered smooth by the formation of oxygen surfactant layers during ultra-low pressure sputtering with Ar as the sputtering gas and the admixture of a small amount of oxygen.